The present invention relates to black iron-based particles and a black toner containing the black iron-based particles, and more particularly, to black iron-based particles having not only an excellent blackness but also a less magnetization value, and a black toner containing the black iron-based particles, which has not only an excellent blackness but also a low magnetization value.
The black iron-based particles of the present invention are used as black-colored pigments and paints, colorants for resin compositions, etc., and, in particular, are applicable to black non-magnetic toners.
Hitherto, black pigments such as magnetite particles, ilmenite particles and carbon black have been generally used as colorants for paints, printing inks, cosmetics, rubbers, resin compositions or the like.
In particular, composite particles prepared by mixing-dispersing black magnetic iron oxide particles such as magnetite particles in resins have been more frequently used for a magnetic toner as an electrophotographic developer.
With recent tendency to high-speed copying operation and high-image quality for laser beam printers or digital copying machines, it has been strongly required to improve properties of a black toner used as a developer. In particular, the black toner has been strongly required to have a sufficient blackness.
Separately, in recent years, with the progress of full color printing and copying techniques, non-magnetic toners have been used in the fields of printers or copying machines.
Thus, in the field of such black toners, it has also been demanded to provide black non-magnetic toners having non-magnetic properties or a much less magnetization value which are suitably applicable to the current developing systems.
Although, as described above, it has been strongly required to improve various properties of the black toners, it is known that the black toners, especially black pigments contained therein, have significant influences on developing characteristics. That is, since the properties of the black toners have a close relationship with those of the black pigments mixed and dispersed therein, it has also been strongly required to further improve properties of the black pigments used in the black toners.
Namely, in order to obtain black toners exhibiting an excellent blackness, black particles used therein have been required to show a sufficient blackness and an excellent dispersibility. Also, in order to suitably apply the black toners to the current developing systems using non-magnetic toners, the black particles used therein have also been required to be particles having non-magnetic properties or a much less magnetization value.
It is known that since carbon black as non-magnetic particles is in the form of fine particles having an average particle diameter of about 0.005 to 0.05 μm, it may be difficult to disperse the carbon black in vehicle or resin compositions. In addition, it is also known that since the carbon black is in the form of bulky particles having a bulk density of about 0.1 g/cm3, the carbon black is poor in handling property and workability.
Thus, it has been required to provide black particles having an excellent blackness and a low magnetization value. The low magnetization value (saturation magnetization value) used herein means that the magnetization value (saturation magnetization value) is minimized so as to be applicable to the current developing systems, more specifically, is usually not more than 60 Am2/kg, preferably not more than 40 Am2/kg.
As the black-colored iron-based particles, there are known ilmenite particles obtained by hydrothermal treatment (Japanese Patent Application Laid-Open (KOKAI) No. 1-298028(1989)); black pigment particles having a mixed composition composed of Fe2TiO5 and Fe2O3—FeTiO3 solid solution (Japanese Patent Application Laid-Open (KOKAI) No. 3-2276(1991)); black magnetic iron oxide particles having a magnetization value σ1000 (at a magnetic field of 1 kOe) of 20 to 50 Am2/kg, and containing titanium in an amount of 0.5 to 10.0 atm % based on whole Fe (Japanese Patent Application Laid-Open (KOKAI) No. 8-34617(1996)); and titanium-containing magnetite particles (Japanese Patent Application Laid-Open (KOKAI) No. 2002-196528).
Although, at present, it has been strongly required to provide black iron-based particles having an excellent blackness and a less magnetization value, such particles have not been obtained conventionally.
That is, in Japanese Patent Application Laid-Open (KOKAI) No. 1-298028(1989), it is described that the ilmenite particles are produced by hydrothermal treatment using Ti3+. However, the production of the ilmenite particles by hydrothermal treatment is disadvantageous from industrial viewpoints.
Although the non-magnetic particles described in Japanese Patent Application Laid-Open (KOKAI) No. 3-2276(1991) contain Fe2TiO5 and, therefore, have a low magnetization value, such particles have a low tinting strength and, therefore, fail to show a sufficient blackness, as shown in the below-mentioned Comparative Examples.
The black magnetic iron oxide particles described in Japanese Patent Application Laid-Open (KOKAI) No. 8-34617(1996) have a Ti content of 0.5 to 10 atm % and a magnetization value σ1000 at a magnetic field of 1 kOe as high as 20 to 50 emu/g and, therefore, fail to sufficiently apply to the current non-magnetic developing systems.
In Japanese Patent Application Laid-Open (KOKAI) No. 2002-196528, there is described that the black toner containing a metal oxide has a magnetization value of not more than 40 emu/g. However, since Ti-containing magnetite particles having a large average particle diameter were used as the metal oxide in Examples, the obtained black toner fails to show a satisfactory tinting strength.
As a result of the present inventors' earnest studies, it has been found the following matters. Upon production of the black iron-based particles, it is important to add a titanium compound to a water suspension of magnetite particles for coating the surface of the magnetite particles with the titanium compound (titanium oxide hydroxide). In order to form the coat of the titanium oxide hydroxide, it is necessary to allow the water suspension to maintain its pH value without lowering thereof, obtained immediately after adding the titanium compound thereto. When the magnetite particles coated with the titanium oxide hydroxide are heat-calcined at a specific temperature under a non-oxidative atmosphere, and then pulverized, the obtained particles can unexpectedly show not only an excellent blackness but also a low magnetization value. The present invention has been attained on the basis of the above findings.